Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Annotations already declare readOnlyHint, openWorldHint=false, idempotentHint and destructiveHint=false, so the safety profile is covered; the description's 'no network access, no stored state' and 'deterministic' largely restate them. It does add genuinely new behavioral data — the per-call limits (64 KB JSON input, 1 s per call, 1 MB result) and the precise output formats — which the annotations do not carry.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Is the description appropriately sized, front-loaded, and free of redundancy?

The labeled sections (Cuándo / No para / Efectos secundarios / Límites) are well front-loaded and scannable, but the entire content is duplicated verbatim in Spanish and English, roughly doubling length for zero additional information per reader.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

An output schema exists, so return-value documentation is unnecessary, and the description covers the remaining gaps an agent needs: the exact output formats, the routing alternative, determinism, and operational limits. Nothing required to invoke it correctly is missing.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema description coverage is 100% with only two parameters, so the schema already documents 'seconds' (range, example) and 'lang' (enum, default). The description adds only the loose mapping that 'lang' governs the words output; it gives no guidance on negative seconds or the 10^12 bound, so the baseline 3 applies.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Does the description clearly state what the tool does and how it differs from similar tools?

States a specific verb and resource (formats seconds as a duration) and enumerates all four output shapes — ISO 8601 days-and-time, compact '1d 2h 3m 4s', clock '26:03:04', and words in English or Spanish. It explicitly separates itself from the sibling duration-parse, so an agent can route without opening either schema.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Does the description explain when to use this tool, when not to, or what alternatives exist?

Provides an explicit 'When:' trigger (exact date, time-zone or calendar results — offsets, DST, month ends, leap years) and an explicit 'Not for:' exclusion naming the alternative tool (duration-parse). This is the when/when-not/alternative pattern at full strength.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.